Ruthenium Complexes with PNN Pincer Ligands Based on (Chiral) Pyrrolidines: Synthesis, Structure, and Dynamic Stereochemistry

We
report the synthesis of lutidine-based PNN type metal pincer
complexes, using achiral (pyrrolidine) as well as chiral ((R,R)-2,5-dimethylpyrrolidine and (R)-2-methylpyrrolidine) substituents at the N side arm of
the pincer ligand. With the six-coordinate saturated Ru pincers (PNN)­Ru­(H)­(CO)­(Cl),
which have an aromatic pyridine ligand backbone, as the starting materials,
treatment with strong base (KOtBu) generated
the corresponding dearomatized pincer complexes (PNN′)­Ru­(H)­(CO).
Spectroscopic, crystallographic, and computational studies demonstrate
that the C-centered chirality from the chiral pyrrolidine group exerts
a small but non-negligible influence on the preferred stereochemistry
at Ru (and N in the case of (R)-2-methylpyrrolidine)
that is reflected in the equilibrium distribution of diastereomers
of these Ru complexes in solution. Our data show that the N- and Ru-based
stereogenic centers in this class of compounds are stereochemically
labile and the mechanisms for epimerization are discussed. Inversion
at the Ru center in the dearomatized complexes is proposed to occur
via a rearomatized Ru(0) intermediate in which the Ru-bound hydride
is transferred to the ligand. Support for this comes from the spectroscopic
characterization of a closely related Ru(0) species that is obtained
by reaction with CO. Testing these catalysts in enantioselective oxa-Michael
addition or transfer hydrogenation led to racemic products, while
a low ee (8%) was observed in the hydrogenation of 4-fluoroacetophenone.
The lack of appreciable enantioinduction with these catalysts is ascribed
to the kinetic lability of the Ru stereocenter, which results in the
formation of equilibrium mixtures in which several diastereomers of
the catalyst are present.